U.K. Pat. No. 724,019 describes a porous panel assembled by mechanical fasteners. A panel of the type described in that patent is shown diagrammatically in FIG. 1 of the accompanying drawings. As will be seen from that Figure, the panel comprises an outer skin 1 of porous metal, a sheet 2 of microporous plastic, edge spacers 3, and a non-porous backing sheet 4 assembled to the outer skin 1 by mechanical fastners 5. A cavity 7 is defined between sheets 2 and 4, and a connection 6 is provided to admit liquid into the cavity 7. Modified forms of this panel are described in U.K. Pat. Nos. 1,094,372 and 1,102,958.